Alloying two semiconductors of different bandgaps generally results in a semiconductor of bandgap different from that of either original constituent. Varying the relative compositions of the two constituents will lead to the corresponding change in bandgap. This has long been one of the standard methods of achieving bandgaps (and thus the operating wavelength of an optical device) that are not provided by naturally-occurring semiconductors. Unfortunately, this method of achieving wavelength variability is severely limited by existing methods of growing planar epitaxial heterostructures of semiconductor thin films on a crystalline substrate. Such methods invariably require a close match of lattice constants of the substrate and the alloy materials to be grown (or a means of relieving the strain due to lattice mismatch). The very limited lattice constant mismatch required for growing high quality wafers has been the main obstacle of making semiconductor-based optoelectronic devices (such as lasers, detectors, multi-color detectors and solar cells) with controllable and widely variable (or tunable) operating wavelengths. With the advent of nanowire based technology, such restrictions are removed or are very much relaxed, depending on the method of growth. For epitaxial growth of nanowires, the relaxed requirement of lattice matching has led to the growth of materials with a mismatch as large as 8%. InP, GaAs, and other III-V nanowires have been epitaxially grown on Si (see, Martensson et al., Nano Lett. 4, 1987-1990 (2004)) and InAs and InP nanowires have been grown into nanowire heterostructures (see, Bjork et al., Nano Lett. 2, 87-89 (2002)) despite large lattice mismatches. In addition, nanowires can also be grown using an amorphous substrate as simply a mechanical support, allowing alloy nanowire growth with a much larger range of composition variation than is possible with planar growth technologies. This has led to the growth of a wide range of alloy compositions between two binary compounds such as CdS and CdSe (see, Pan et al., J. Am. Chem. Soc. 127, 15692 (2005); Pan et al., Nanotech. 17, 1083-1086 (2006)), InN and GaN (see, Kuykendall et al., Nature Materials 6, 951-956 (2007)), ZnS and CdS (see, Liu et al., Adv. Mater. 17, 1372-1377 (2005)), ZnSe and CdSe (see, Shan et al., Appl. Phys. Lett. 87, 033108 (2005)), ZnO and MgO (see, Lu et al., Appl. Phys. Lett. 91, 193108 (2007)), and so on. Such controllable alloy composition variation provides unprecedented access to new wavelength ranges using semiconductor alloys. In this connection, it is worth mentioning that semiconductor nanoparticle (nanocrystal) technology can also provide large range of wavelength flexibility by either alloying or size variation. But for optical applications, nanowire alloys have an additional advantage not available with nanoparticles, since individual nanowires intrinsically provide both channels for electronic conduction and waveguides for optical devices, in addition to serving as a gain/absorbing material. This is why individual nanowires have been shown to act as nanolasers (see, Huang et al., Science 292, 1897-1899 (2001); Johnson, J. C. et al. J. Phys. Chem. B. 105, 11387-11390 (2001); Duan et al., Nature 421, 241-245 (2003); Agarwal et al., Nano Lett. 5, 917-920 (2005); Zapien et al., Appl. Phys. Lett. 84, 1189-1191 (2004); Chin et al., Appl. Phys. Lett. 88, 163115 (2006); Maslov et al., Appl. Phys. Lett. 83, 1237-1239 (2003); Maslov et al., IEEE J. Quant. Elect. 40, 1389-1397 (2004)).
Although various semiconductor alloy nanowires of different compositions have been achieved under separate growth conditions, it is both challenging and important to achieve a full-range composition variation within a single substrate in a single run of growth. To achieve fully tunable lasing within the entire composition range on a single substrate would be even more appealing and challenging.
Therefore there exists a need in the art to provide semiconductor compositions and structural assemblies which enable continuously tunable bandgap on a single substrate. Further, methods for preparing the same which enable straightforward preparation of such semiconductor structures in a single step are needed in the art.